Abstract

The risk of transmission of ruminant transmissible spongiform encephalopathy (TSE) to humans was thought to be low due to the lack of association between sheep scrapie and incidence of human TSE. However a single TSE agent strain has been shown to cause both bovine spongiform encephalopathy (BSE) and human vCJD, indicating that some ruminant TSEs may be transmissible to humans. While the transmission of cattle BSE to humans in transgenic mouse models has been inefficient, indicating the presence of a significant transmission barrier between cattle and humans, BSE has been transmitted to a number of other species. Here we aimed to further investigate the human transmission barrier following passage of BSE in a sheep. Following inoculation with cattle BSE, gene targeted transgenic mice expressing human PrP showed no clinical or pathological signs of TSE disease. However following inoculation with an isolate of BSE that had been passaged through a sheep, TSE associated vacuolation and proteinase-K resistant PrP deposition were observed in mice homozygous for the codon 129-methionine PRNP gene. This observation may be due to higher titres of the BSE agent in sheep, or an increased susceptibility of humans to BSE prions following passage through a sheep. However these data confirm that, contrary to previous predictions, it is possible that a sheep prion may be transmissible to humans and that BSE from other species may be a public health risk.